Direct eutectic AuSn solder bumping on Al bond pad surface using laser solder ball jetting

Au-rich eutectic AuSn (Au80wt%-Sn20wt%) solder ball alloy is extensively used in MEMS and optoelectronics packaging, for providing flip-chip solder bump interconnections. In this paper, we will look into the possibility of using laser solder ball jetting process for direct eutectic AuSn solder bumping on Al bond pad surface, and compare with eutectic AuSn solder bumping on Al bond pad with Ti/Ni/Au UBM structure. The laser jetted eutectic AuSn solder bumps were observed to wet and form hemi-spherical bumps on the Al bond pad surface, with and without UBM structure. FIB-EDX analysis of the laser jetted eutectic AuSn solder bump on Al bond pad with UBM structure showed formation of dense islands of Au₅Sn IMC layer from the top Au finishing layer of the UBM structure. On the other hand, only a few clusters of Au₅Sn IMC were formed near to the solder joint of the laser jetted eutectic AuSn solder bump on Al bond pad surface. Ball shear test on the laser jetted eutectic AuSn solder bumps exhibited average solder shear strength of 4.52g/mil² and 14.22g/mil², on Al bond pad surface and Al bond pad with UBM structure respectively. Laser jetted eutectic AuSn solder bumps on Al bond pad surface displayed pad lift failure mode, as compared to failure at Al bond pad layer for Al bond pad with UBM structure. In conclusion, eutectic AuSn solder balls could be bumped onto Al bond pad surface via laser jetting.